Electrostatically charged cryogen fog

ABSTRACT

An apparatus for providing electrostatically charged cryogen fog to a product includes an injection tube having a chamber therein for receiving a cryogenic substance introduced into the chamber and to be provided to the product, a first inlet in communication with the chamber for providing the cryogenic substance to the chamber, an electrostatic device in communication with the chamber for charging the cryogenic substance with a charge opposite to a charge of the product, and an outlet in communication with the chamber and through which the charged cryogenic substance is exhausted to be attracted to the product for adhesion thereto. A method of providing an electrostatically charged nitrogen fog to a product is also provided.

BACKGROUND

The present embodiments relate to chilling or freezing of products suchas food products, with a cryogen substance.

In known cryogen food freezing systems where nitrogen is used, liquidnitrogen is sprayed into a freezing chamber where the products aredisposed to provide refrigeration for the products. The spray of liquidnitrogen is directed onto the warm surface of the incoming food productso that a phase change (heat of vaporization) occurs at the surface ofthe product. This evaporative cooling effect creates extremely high heattransfer coefficients for the food product to chill or freeze same.

Unfortunately, it is extremely difficult if not impossible to have anacceptable proportion of the liquid cryogen contact the food product, asthe cryogen upon discharge from the nozzles is sprayed into the chambertoward the food products positioned on a conveying belt where 25-60% ofliquid cryogen is lost. A large proportion of the liquid nitrogencontacts the belt, instead of the food product, and indeed passesthrough the mesh belt or contacts other areas of the spray chamber inwhich the food product is being conveyed. This results in a waste ofrefrigeration which would otherwise have been achieved during the heatof vaporization of the liquid nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, referencemay be had to the following drawing figures taken in conjunction withthe description of the embodiments, of which:

FIGS. 1 and 2 show an apparatus and method for providing anelectrostatiscally charged nitrogen fog.

DETAILED DESCRIPTION OF THE INVENTION

The present embodiments provide an apparatus and method for coating aproduct, such as for example, a food product, with a cryogen, such asfor example a nitrogen (N₂) or carbon dioxide (CO₂) fog using anelectrostatic charge. When, for example, liquid nitrogen is atomizedeither by electrostatic atomization or by an ultrasonic fog generator,surfaces of the nitrogen droplets can be charged by exposure to anelectrostatic field. The product, and by way of example a food product,can be grounded or given an opposite charge which will attract theatomized nitrogen droplets, thereby allowing for a much greaterproportion of the cryogenic spray to be deposited onto the food productwhich will maximize overall evaporative heat transfer at the foodproduct. In effect, the opposite charges of the atomized spray and thefood product provide for attraction of the spray to the food product tobetter control the amount of cryogenic spray used, and to be confidentthat any nitrogen spray used will indeed contact the food product andnot be wasted by adhering or passing through the conveyor belt or othersurfaces of the conveyor chamber through which the food product is beingtransferred for chilling or freezing.

Using a food product for example, electrostatic atomization exposes theliquid cryogen to an intense electric field between a charged atomizerand a grounded food product. The charge transfers to the liquid cryogen,and repulsive forces between the atomizer and the liquid cryogen forcedroplets of the cryogen from the atomizer and direct them toward thefood product. An energy source for the electrostatic atomization is anelectric charge received by the liquid cryogen. Particle size andelectrostatic atomization of the liquid cryogen is a function of three(3) factors, that is (i) electric field strength, (ii) liquid flow rate,and (iii) fluid properties (including electrical properties of thefluid).

Referring to FIGS. 1 and 2, and using nitrogen (N₂) for the cryogenicliquid by way of example only, an apparatus for generating anelectrostactically charged nitrogen fog is shown generally at 10 andincludes a liquid nitrogen (N₂) injection tube 12 or nozzle Theinjection tube 12 is connected to a DC voltage generator 16. A conveyorbelt 18 is spaced apart from and disposed beneath the outlet 14 of theinjection tube 12, and provides a surface 20 upon which a product 22,such as a food product for example, is disposed for transport beneaththe injection tube 12. The conveyor belt 18 is fabricated from metal andmay be solid or of the mesh-type. A ground wire 24 is in contact withthe conveyor belt 18 to ground said belt, including the belt surface 20.

The injection tube 12 includes an internal chamber 26 accessible by aninlet 28 or inlet port. Droplets 30 of nitrogen are produced external tothe injection tube 12 and provided to same via the inlet 28 as shown byarrow 32. The injection tube 12 also includes another inlet 34 or inletport in communication with the chamber 26 so that gaseous nitrogen canbe introduced into the chamber as indicated by arrow 36. The gaseousnitrogen is a carrier gas 35 for the droplets 30. If carbon dioxide isused for the droplets 30 then the carrier gas will also be carbondioxide. The carrier gas 35 can also be provided to atomize the droplets30 introduced into the injection tube 12 upstream of the outlet 14.

The DC voltage generator 16 has a portion 17 extending into the internalchamber 26. The portion 17 can be of any length and therefore have anyamount of surface area. However, by maximizing the surface area of theportion 17 for exposure to the cryogen droplets this will increase thenumber of charged particles 40 for adhering to the product 22. Theinjection tube 12 includes a side wall 38 which is insulated or vacuumjacketed. The outlet 14 of the injection tube 12 may be in registrationwith the product 22 passing beneath the outlet 14 as further discussedbelow.

Atomization of the liquid cryogen, such as liquid nitrogen, can occur ina plurality of ways. Such atomization can occur by (1) electrostaticatomization wherein the electric field created by the DC generatoractually atomizes the liquid nitrogen into droplets to be attracted tothe product 22, or (2) the liquid nitrogen can be atomized upstream ofthe outlet 14 and upon entry into the chamber such atomized particlesare charged prior to being sprayed from the outlet 14 onto the product22.

The nitrogen droplets 30 introduced through the inlet 28 into thechamber 26 are “un-charged” nitrogen particles. Upon exposure to thehigh voltage DC current provided by the generator 16, the nitrogenparticles receive a charge transfer and become negatively chargedparticles 40. Free ions 42 are also provided upon discharge from theoutlet 14.

Because the conveyor belt 18 is grounded, the food product 22 is alsogrounded. Accordingly, because the food product 22 is disposed closer tothe outlet 14 of the injection tube 12 than is the conveyor belt 18, thecharged nitrogen particles 40 in an atomized state will first adhere tothe food product before adhering to the underlying conveyor belt. Ineffect, the food product 22 becomes a magnet for the charged nitrogenparticles 40 which are attracted to the food product to adhere to thefood product's exterior surface. Therefore, a majority of the atomizedcharged nitrogen particles 40 adhere to the exterior surface of the foodproduct 22 as opposed to the underlying conveyor belt 18. There istherefore less waste of the nitrogen particles and a greater percentageof same is used for heat transfer effect at the food product 22.

The embodiments provide for an increase in overall heat transfer effectof the food freezing process and provide for a more efficient use of theatomized cryogen. It is also possible to use liquid carbon dioxide (CO₂)instead of liquid nitrogen with the present embodiments.

It will be understood that the embodiments described herein are merelyexemplary, and that one skilled in the art may make variations andmodifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as described and claimedherein. Further, all embodiments disclosed are not necessarily in thealternative, as various embodiments of the invention may be combined toprovide the desired result.

1. An apparatus for providing electrostatically charged cryogen fog to aproduct, comprising an injection tube having a chamber therein forreceiving a cryogenic substance introduced into the chamber and to beprovided to the product, a first inlet in communication with the chamberfor providing the cryogenic substance to the chamber, an electrostaticdevice in communication with the chamber for charging the cryogenicsubstance with a charge opposite to a charge of the product, and anoutlet in communication with the chamber and through which the chargedcryogenic substance is exhausted to be attracted to the product foradhesion thereto.
 2. The apparatus of claim 1, wherein the productcomprises a food product.
 3. The apparatus of claim 1, wherein thecryogenic substance comprises liquid droplets selected from carbondioxide and nitrogen.
 4. The apparatus of claim 1, wherein the injectiontube further comprises a second inlet in communication with the chamberfor providing a carrier gas to the cryogenic substance to be exhaustedat the outlet.
 5. The apparatus of claim 4, wherein the carrier gascomprises a gaseous equivalent of the cryogenic substance.
 6. Theapparatus of claim 1, wherein the electrostactic device comprises aportion disposed in the chamber, the portion having a surface areaexposed to the cryogenic substance.
 7. The apparatus of claim 1, whereinthe injection tube comprises insulation at an exterior surface thereof.8. The apparatus of claim 1, wherein the injection tube comprises anozzle.
 9. A method of providing an electrostatically charged cryogenfog to a product, comprising charging a cryogenic substance with a firstcharge, providing a product with a second charge opposite to the firstcharge and in relative proximity to the cryogenic substance, repulsingthe cryogenic substance with the first charge to be attracted to theproduct having the second charge, and attracting the cryogenic substanceto the product for adhering thereto.
 10. The method of claim 9, whereinthe product comprises a food product.
 11. The method of claim 9, whereinthe cryogenic substance comprises liquid droplets selected from carbondioxide and nitrogen.
 12. The method of claim 9, further comprisingproviding a carrier gas to the cryogenic substance for carrying thecryogenic substance from the charging to the adhering of the cryogenicsubstance to the product.
 13. The method of claim 12, wherein thecarrier gas comprises a gaseous equivalent of the cryogenic substance.14. The method of claim 9, further comprising atomizing the cryogenicsubstance after repulsing the cryogenic substance.
 15. The method ofclaim 9, further comprising atomizing the cryogenic substance beforecharging the cryogenic substance.